1. Field of the Invention
The present invention relates to a sputtering method and a sputtering apparatus, which are implemented while monitoring the emission intensity of plasma. The present invention develops a superior effect particularly when forming a deposit film, such as a transparent conductive film, by a reactive sputtering process.
2. Description of the Related Art
In general, two methods are proposed for producing a transparent conductive film made of a metal oxide on a substrate by a sputtering process, i.e., one in which an oxide, such as In2O3xe2x80x94SnO2, is employed as a target and is subjected to sputtering in an Ar gas, and the other (reactive sputtering process) in which an alloy, such as Inxe2x80x94Sn, is subjected to sputtering in a gas mixture of an inert gas, such as Ar, and O2.
The former method is able to produce a film having low electrical resistance and high transmittance by sputtering, but has difficulty in increasing a film forming rate.
On the other hand, the latter reactive sputtering process is able to increase a film forming rate. It is reported that, particularly in a DC magnetron sputtering apparatus using a cylindrical rotating target disclosed in U.S. Pat. No. 4,356,073 and U.S. Pat. No. 4,422,916, the efficiency in utilization of a target material is about 2.5 to 3 times as large as that obtainable with a conventional planar type apparatus (xe2x80x9cFunctional Materialsxe2x80x9d, Mar., 1991, Vol. 11, No. 3, pp. 35-41).
The reactive sputtering process using the rotating target has advantages in that a target material can be saved and a production down time due to target replacement can be greatly reduced.
The DC magnetron sputtering apparatus using the rotating target is therefore suitable for mass-production.
In the reactive sputtering process, however, a film forming condition, in particular, a gas flow rate, has a very narrow suitable range, and hence a difficulty occurs in control of film forming parameters, e.g., uniformity in sheet resistance and transmittance, discharge stability and so on, when a transparent conductive film is formed on, e.g., a sheet-like substrate having a large area.
As an improvement for overcoming the problem mentioned above, there is known a reactive sputtering process employing a plasma emission monitor (abbreviated as xe2x80x9cPEMxe2x80x9d hereinafter) (S. Schiller, U. Heisig, Chr. Komdorfer, J. Strumpfel, and V. Kirchhoff, xe2x80x9cProgress in the Application of the Plasma Emission Monitor in Web Coatingxe2x80x9d, Proceedings of the 2nd International Conference on Vacuum Web Coating, Fort Lauderdale, Fla., USA October 1988).
The PEM is a device for collecting plasma emissions by a collimator, introducing the emissions to a photomultiplier through a spectroscope, and monitoring a plasma state with an electrical signal obtained through photoelectric conversion of the emissions by the photomultiplier. In a sputtering apparatus employing a PEM, the function of adjusting the flow rate of an introduced reactive gas and holding the emission intensity of plasma constant can be developed with the sensitivity of the photomultiplier of the PEM set to a certain value.
Also, Japanese Patent Laid-Open No. 11-29863 discloses a technique for forming an ITO (Indium Tin Oxide) film on a substrate. A gist of the disclosed technique is as follows. A substrate is set in a film forming chamber, and a discharge is generated in the film forming chamber under a condition in which a sputtering gas is introduced, but a reactive gas is not introduced to the chamber. The sensitivity of a device for monitoring the emission intensity of discharge plasma is adjusted and the amount of the introduced reactive gas is controlled so that the plasma emission intensity is adjusted at a predetermined value. Then, a target is subjected to sputtering while a set value of the emission intensity is varied so as to hold the actual emission intensity of plasma constant at all times. As a result, a deposition rate is held constant.
In other words, according to the disclosed technique, when forming an ITO film, a deposition rate in the formation of the film is held constant by adjusting the flow rate of an introduced reactive gas (O2) so that the actual-emission intensity of In plasma (wavelength=451.1 nm) is held constant.
With the techniques described above, it has become possible to produce a fairly satisfactory deposit film at a certain level of stability using the reactive sputtering process.
However, when continuously forming a deposition film of superior characteristics at a high film forming rate for a long time, the following problems still remain not yet overcome.
With recent progress of the technology, levels of characteristics and uniformity required for a deposit film have increased year by year. Correspondingly, severer criteria than those ones, which have been employed so far, are applied for determination of characteristics and uniformity. Also, from the viewpoint of cost effectiveness, a deposit film forming method and apparatus with higher productivity is demanded.
Under such a situation, when the above-described conventional reactive sputtering process combined with the PEM is employed to form a transparent conductive film on a substrate for a long time using an Inxe2x80x94Sn alloy target, the film forming rate changes over time, whereupon uniformity in sheet resistance and transmittance deteriorates despite of the PEM control. This may result in that the formed film does not satisfy the criteria required as a product (or the yield reduces).
In particular, when sputtering is performed for a long time using a metal having a low melting point, such as In, the surface shape (e.g., asperities) and the surface condition (e.g., partial melting of an outermost surface) of a target change over time. Eventually, even though the emission intensity is controlled by the PEM so as to fall within a certain range, it is often inevitable that the film thickness, the sheet resistance and the transmittance become not uniform.
Those problems are more critical particularly when forming a deposit film for a long time using a long-strip substrate as practiced by the so-called roll-to-roll method, or when increasing, e.g., the applied power and/or the flow rate of an introduced gas during sputtering to increase a deposit film forming rate, because the surface shape and the surface condition of a target are changed to a larger extent in a short time.
It is one object of the present invention to provide a sputtering method and a sputtering apparatus, which can hold constant the sheet resistance and the transmittance of a deposit film (particularly a transparent conductive film), and which can easily control a film forming rate to be kept constant. The sputtering method and apparatus of the present invention is particularly suitable for use in the case of performing reactive sputtering with a target using a metal having a low melting point, such as an Inxe2x80x94Sn alloy.
Another object of the present invention is to provide a sputtering method and a sputtering apparatus, which can form a deposit film having high uniformity in film thickness, sheet resistance and transmittance and having superior characteristics, particularly even in the case of forming a deposit film (especially a transparent conductive film) for a long time using an long-strip substrate or increasing a deposit film forming rate.
As a result of conducting intensive studies on a reactive sputtering process with a DC magnetron sputtering apparatus using a cylindrical rotating target while attention is especially focused on the surface condition, the sputtering rate and the surface reaction of the target, the inventors have accomplished the present invention having features as follows.
According to one aspect of the present invention, there is provided a sputtering method for forming a film on a substrate in a film forming space while monitoring emission intensity of plasma, the method comprising the steps of detecting a thickness of the film formed on the substrate; comparing a detected value with a preset value of the film thickness; and deciding a target value of the emission intensity in accordance with a compared result.
In the sputtering method of the present invention, preferably, a flow rate of at least one of gases introduced to the film forming space is controlled such that actual emission intensity is adjusted at the target value of the emission intensity.
The target value of the emission intensity is preferably set to fall in a predetermined range defined beforehand. Furthermore, preferably, if the target value deviates from the predetermined range, sputtering is stopped.
In the sputtering method of the present invention, a target containing In may be employed as a sputtering target. Also, a cylindrical rotating target is preferably employed as a sputtering target.
In the sputtering method of the present invention, preferably, an oxygen gas is selected as one of the gases, of which flow rate is controlled.
According to another aspect of the present invention, there is provided a sputtering apparatus comprising a film forming container, a substrate feeding mechanism, and an emission intensity monitor, the apparatus further comprising a film thickness measuring device for measuring a thickness of a film formed on a substrate and outputting a measured result; and a comparator for comparing an output of the film thickness measuring device with a preset value of the film thickness and outputting a target value of the emission intensity monitor in accordance with a compared result.
Preferably, the sputtering apparatus of the present invention further comprises a gas flow rate control mechanism for receiving the target value of the emission intensity monitor and controlling a flow rate of at least one of gases introduced to the film forming container in accordance with the target value.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.